1. Field of the Invention
This invention resides in the field of corrosion testing of steel and other metals, and the use of steel reinforcements and other metals embedded in concrete and cementitious material in general.
2. Description of the Prior Art
Concrete is widely used as a construction material due to the ease with which it can be transported, formed and applied as well as its durability and high compressive strength. Concrete has a low tensile strength, but is readily corrected by the use of steel reinforcing bars. Steel-reinforced concrete has been used successfully in roadways, bridges, parking structures, and other constructions where high tensile strength is needed. These constructions proved reasonably durable until the late 1960""s, when premature concrete delamination and spalling, previously encountered only in coastal areas, began to occur in regions in the snow belt. This coincided with the increased use of deicing salts on roads and bridges, and one result was that reinforced concrete bridge decks in those regions began to require maintenance after as little as five years of service. It was ultimately recognized that the low performance was caused by corrosion of the reinforcing steel in the concrete due to the intrusion of small amounts of chloride into the concrete.
The cost of maintaining bridge decks is very high and rising, but the use of chloride-containing deicing salts is increasing as well. Alternative deicing agents such as calcium magnesium acetate are available, but the higher cost of these agents has prevented them from having a significant effect on the widespread use of chloride salts. Federal agencies such as the Structure Division of the Federal Highway Administration (FHWA) are thus continually seeking cheaper yet still effective ways of reducing the chloride-induced corrosion of steel-reinforced concrete structures.
Concrete and other cementitious materials are highly alkaline, due to the presence of a large amount of calcium hydroxide and small amounts of alkali elements such as sodium and potassium. The high alkalinity causes the embedded steel to form a thin film along its surface. The film, which may consist of an inner dense spinel phase (Fe3O4/xcex3-Fe2O3) in epitaxial orientation to the steel substrate and an outer layer of xcex1-iron oxyhydroxide (xcex1-FeOOH), protects the steel from chemical attack but is itself susceptible to attack by chloride ions. This exposes the underlying steel, rendering it vulnerable to attack by the oxygen and moisture present in the concrete at the concrete-steel interface. Analyses have shown that corrosive attack can occur both in the presence and absence of oxygen.
Methods of preventing or retarding the corrosion of steel-reinforcements have included the use of epoxy coatings over the steel reinforcement bars and the use of low-permeability concretes such as low water-cement ratio Portland cement concrete, latex-modified concrete, and other specialty concretes. The use of an increased concrete cover over the reinforcing steel has also been used, as have waterproof membranes and asphalt overlays. In general, however, coatings and overlays are either impractical or only serve to slow down the corrosion rate rather than stopping it entirely. Uncoated steel bars are still the most widely used.
Other materials are often embedded in concrete to serve a variety of functions. Examples are metallic conduits for conveying liquids, gases, or electricity. Water pipes, for example, which can be copper, brass of other non-ferrous metals as well as ferrous metals themselves are at times embedded in concrete and raise corrosion questions. Electrical conduits and wires for either transmitting electricity or grounding are also used and raise similar corrosion questions. In all cases, standardized testing methods for evaluating the chloride corrosion threshold are a useful means of evaluating the metals and the concrete in which they are embedded.
In the case of steel-reinforced concrete, it was reported in 1962 that the chloride corrosion threshold, which is defined as the lowest concentration of chloride in the concrete immediately surrounding the steel that will initiate corrosion of the steel, is 0.15 percent soluble chloride, by weight of cement. Lewis, D. A., xe2x80x9cSome Aspects of the Corrosion of Steel in Concrete,xe2x80x9d Proceeding of the First International Congress on Metallic Corrosion, London, 1962, pp. 547-555. Studies at the FHWA laboratories in the mid-1970""s led to an even lower estimate of the chloride corrosion threshold, this time expressed as total chloride, which includes both ionized and organic chloride. The FHWA estimate was 0.033 percent total chloride (again by weight of concrete). Berman, H. A., xe2x80x9cThe Effects of Sodium Chloride on the Corrosion of Concrete Reinforcing Steel and on the pH of Calcium Hydroxide Solution,xe2x80x9d Report No. FHWA-RD-74-1, Federal Highway Administration, Washington, D.C., 1974; Clear, K. C., xe2x80x9cTime-to-Corrosion of Reinforcing Steel in Concrete Slabs,xe2x80x9d Report No. FHWA-RD-76-70, Federal Highway Administration, Washington, D.C., 1976. Raising the chloride corrosion threshold lowers the susceptibility of the steel to chloride-induced corrosion, and the value of the threshold has indeed been shown to vary with the type of cement or cementitious material and the mix proportions used, since these can entail differences, for example, in the concentrations of tricalcium aluminate and hydroxide ion.
The testing of the chloride corrosion of embedded uncoated steel is a valuable means of evaluating cementitious materials and cementitious material admixtures as well as the steel itself. This is also true for other embedded metals. Typical of the commonly used methods to test the chloride corrosion of embedded steel is ASTM G 109, xe2x80x9cStandard Test Method for Determining the Effects of Chemical Admixtures on the Corrosion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments,xe2x80x9d published by the ASTM Committee G-1 on Corrosion, Deterioration, and Degradation of Materials, Subcommittee G01.14 on Corrosion of Reinforcing Steel, published yearly to the present. This is a static test that entails repeated measurements at four-week intervals and typically over a year to complete. Such an extended time scale severely limits the rate at which new materials can be developed and investigated.
It has now been discovered that the chloride corrosion threshold of metal embedded in a cementitious structure can be determined in an accelerated manner by using an electric field to increase the rate of migration of chloride ions into the structure from an external liquid solution, monitoring an appropriate parameter at the interface between the embedded metal and the cementitious material to determine when corrosion has begun, and then determining the chloride content of the cementitious material at or very close to (i.e., within a few millimeters of) the interface. The electric field that accelerates the chloride ion migration is achieved by placing a chloride ion-containing liquid solution in contact with the structure and applying an electric potential between a cathode immersed in the solution and an anode embedded in the cementitious structure at a location in proximity to the embedded metal. The parameter that is used to detect the onset of corrosion is any of various electrical parameters that bear a known correlation to the corrosion rate. One such parameter is the electric potential at the interface of the metal and the cementitious material, which is monitored by connecting the embedded metal to electric circuitry as a working electrode and applying an electric potential between it and a counter electrode. Another is the polarization resistance, i.e., the slope of the potential vs. the applied current. Preferably, a reference electrode is included that is positioned within a few millimeters, most preferably within about one millimeter, from the interface, and the potential drop between the interface and the reference electrode is measured and expressed as the current density at the interface. A critical value of the current density is recognized as indicative of active corrosion of the metal.
This invention thus resides in a method for determining the chloride corrosion threshold of a steel reinforcement in steel-reinforced concrete and other cementitious material, or of any potentially corrodible metal embedded in a set mass of cementitious material, by the procedure described above. The invention also resides in apparatus for performing the determination, the apparatus including a mold for molding the cementitious material around a sample of the metal, a liquid reservoir or receptacle that can be adjoined to the mold such that the liquid contents of the reservoir are in contact with the cast cementitious material, and various electrodes and electrical connections that permit the electric potentials in the preceding paragraph to be applied and the measurements taken.
The method and apparatus of this invention are useful in obtaining an accelerated chloride corrosion test that can be performed over a time period of as little as 2 to 3 weeks rather than requiring months or years as do conventional chloride corrosion test procedures. The method and apparatus are also useful in comparing different steel alloys as reinforcement bars (or different metals in general), different cementitious material formulations, including the use of mineral and/or chemical admixtures, and different mixing and curing procedures. The information obtained from the practice of this invention can be used for analytical purposes in the development of new or existing materials of construction or as a means of comparing or predicting the service life of a reinforced concrete structure or any embedded metals in a concrete structure.
Other features, embodiments, objects, and advantages of the invention will become apparent from the description that follows.